In the past few years, the etching of semiconductor substrates, dielectric materials and plastic substrates has been marked by a shift from wet chemical to what is known as dry or plasma etch processes. Such "dry" processes, i.e. processes carried out in a vacuum, which are used primarily in semiconductor technology but also in the processing of plastic materials, require an accurate temperature control. Thus, for example, etch processes, such as PE (plasma etching), RIE (reactive ion etching), ECR (electron cyclotron resonance plasma etching), MRIE (magnetic confinement reactive ion etching), triode etching, CAIBE (chemically assisted ion beam etching), photon assisted etching, and the like, necessitate cooling to prevent damage to the substrates to be etched or the films, such as photoresist films, arranged thereon.
Therefore, substrate holders are generally heated to a moderate temperature by scavenging them with liquids, such as water or special oils. Heat transfer between the substrate and the substrate holder is, however, impeded by the gap existing therebetween, which has led to a number of suggestions as to how this might be remedied. U.S. Pat. No. 4,282,924, for example, describes an apparatus for ion implantation, wherein the substrates are pressed against the convexly curved surface of a substrate holder by means of a ring or clamps, and heat transfer between the substrates and the substrate holder, through which the cooling liquid flows, is improved by a thin intermediate layer of an elastomer. U.S. Pat. No. 4,399,016 describes a substrate holder, wherein contact to the substrate is improved by the latter being fixedly attached by electrostatic attraction between the substrate and the substrate holder electrically insulated therefrom. Heat transfer can also be improved by filling the gap between the substrate and the substrate holder with a liquid metal (U.S. Pat. No. 4,129,881).
During vacuum treatment, an article is best cooled by introducing a gas with a high thermal conductivity between the article to be treated, such as, a semiconductor wafer, and the substrate holder. Suitable gases are nitrogen, neon, hydrogen and, in particular, helium (U.S. Pat. No. 4,514,636and many others). Effective heat transfer between the semiconductor wafer and the substrate holder is ensured only if a static gas pressure ranging from about 1.33 to 13.3 mbar, preferably of about 1.33 mbar, is maintained between the two elements. This, in turn, requires that the substrates to be treated be mechanically or electrostatically fixed to the liquid-cooled substrate holder and be sealed from the reaction chamber to prevent an ingress of heat-transferring gas which might noticeably affect the production result.
The previously-described apparatus and methods require mechanically stable substrates which may be readily clamped and sealed from the reaction chamber. In any of these cases, heat transfer is effected by heat conductance in the material or by convection in gas, with the uniformity of cooling depending respectively on the support of the substrate on the substrate holder and the homogeneity and pressure of the heat-conducting gas. Only in a few special cases will the entire substrate surface rest on the substrate holder. Normally, the support will be a multi-point support which poses no problems where the etch rate, as, for example, during plasma or reactive ion etching, is independent of temperature and/or where the thermal conductivity of the substrates to be etched is very high, also ensuring a uniform substrate temperature.
There are, however, substrate materials whose etch rate is strongly temperature-dependent. This holds in particular for plastic substrates. Plastic materials frequently have a low thermal conductivity so that highly irregular etch rates may occur if the substrates are fixed and cooled as described in the previously-cited prior art. Thus, for instance, etch rate differences of more than a factor of two have been detected within a few square centimeters of substrate surface during the reactive ion etching of substrates of polyoxymethylene homo- or copolymers, leading to poor yield.